Analysis of the structure and dynamics of single macromolecules in a fluid sample has attracted considerable interest due in part to the rapid development of methodologies for the manipulation and detection of single macromolecules. For example, recent developments in experimental techniques and available hardware have increased dramatically the sensitivity of detection so that optical detection can be made of single dye molecules in a sample. Single dye detection can be done in an aqueous solution, at room temperature (see, e.g., Weiss, 1999, Science 283: 1676-1683), and in very small volumes to reduce background. Such single-molecule based analytical methods are especially useful in the analysis of biological macromolecules, such as nucleic acid molecules and proteins. Single-molecule analytical methods require small amounts of sample, thereby alleviating tedious efforts in generating large amounts of sample material. For example, single-molecule analytical methods may allow analysis of the structure of nucleic acid molecules without amplification, e.g., by polymerase-chain reaction (PCR). Single-molecule analytical methods also allow analysis of individual molecules, and are thus particularly useful in the identification of structural and/or dynamical features without the effect of averaging over a heterogeneous population.
A single-molecule electrophoresis (SME) method which combines single molecule detection and electrophoresis has been reported for the detection and identification of single molecules in solution (Castro and Shera, 1995, Anal. Chem. 67: 3181-3186). In SME, sizing of single molecules is accomplished through determination of electrophoretic velocities by measuring the time required for individual molecules to travel a fixed distance between two laser beams. This method has been applied to DNA, to fluorescent proteins and to simple organic fluorophores. For example, SME offers a single-molecule method for sizing of DNA restriction fragments. However, SME detects only the presence or absence of a molecule. The method does not provide information regarding the internal structure of a molecule.
A single-molecule DNA sizing method using a microfabricated device has also been reported (Chou et al., 1999, Proc. Natl. Acad. Sci. USA 96:11-13). The method makes use of the fact that the amount of intercalated dye is proportional to the length of the molecule, and determines the lengths of single DNA molecules by measuring the total fluorescence intensity of DNA stained with intercalating dye molecules. Thus, the method does not use electrophoretic mobilities to determine sizes of molecules. This method also does not provide information regarding the internal structure of a molecule.
PCT Publication No. WO 98/10097 discloses a method and apparatus for detection of single molecules emitting two-color fluorescence and determination of molecular weight and concentration of the molecules. The method involves labeling of individual molecules with at least two fluorescent probes of different emission spectra. Simultaneous detection of the two labels indicates the presence of the molecule. The velocity of the molecule is determined by measuring the time required for the molecules to travel a fixed distance between two laser beams. Comparison of the molecule's velocity with that of standard species permits determination of the molecular weight of the molecule, which may be present in a concentration as small as one femtomolar.
Other techniques for characterizing single macromolecules include a method described in U.S. Pat. No. 5,807,677 for direct identification of a specific target nucleic acid sequence having a low copy number in a test solution. This method involves the preparation of a reference solution of a mixture of different short oligonucleotides. Each oligonucleotide includes a sequence complementary to a section of the target sequence and is labeled with one or more fluorescent dye molecules. The reference solution is incubated with the test solution under conditions favorable to hybridization of the short oligonucleotides with the nucleic acid target. The target sequence is identified in the solution by detection of the nucleic acid strands to which one or more of the labeled oligonucleotides are hybridized. To amplify the fluorescence signal, a “cocktail” of different oligonucleotides is used. In this cocktail, the oligonucleotides are capable of hybridizing with sequences adjacent to but not overlapping with the target sequence. The disadvantage of this method is that, in order to design probes of the proper sequence, the exact sequence of the target nucleic acid and surrounding sequences must be known. A method described in U.S. Pat. No. 5,599,664 and European Patent No. EP 0391674 allows sizing of DNA molecules by first subjecting a DNA molecule to a force such that the DNA molecule is elongated and then measuring the conformational relaxation dynamics. In another method (Schmalzing et al., 1998, Analytical Chemistry 70:2303-2310; Schmalzing et al, 1997, Proc. Natl. Acad. Sci. USA 94:10273-10278), microfabricated devices for DNA analysis were developed, including sequencing, which employ small-scale versions of traditional techniques, such as electrophoresis.
None of these single molecule analytical methods allows the determination of the internal structure of the molecule. A challenge to the characterization of the internal structure, e.g., the linear sequence of monomers, in a single polymer chain is the natural tendency of polymers in most media to adopt coiled conformations. The average degree of such coiling is dependent on, inter alia, the interaction of the polymer with the surrounding solution, the rigidity of the polymer, and the energy of interaction of the polymer with itself. In most cases, the coiling is quite significant. For example, a λ-phage DNA, with a B-form contour length of about 16 μm long, has a random coil diameter of approximately 1 μm in water (Smith et al., 1989, Science 243:203-206).
Methods of elongating DNA molecules by fluid flow have been reported (Perkins et al. Science 276:2016-2021; Smith et al., Science 283:1724-1727). In one method, DNA molecules are stretched by an elongational flow. The probability distribution of molecular extension was determined as a function of time and strain rate. Detailed dynamics of elongated DNA molecules in elongational flow has also been observed. In another method DNA molecules are stretched by a steady shear flow. The probability distribution for the molecular extension was determined as a function of shear rate. It was found that, in contrast to the behavior in pure elongational flow, the average polymer extension in shear flow does not display a sharp coil-stretch transition.
DNA has also been stretched by electrophoresis as part of a near-field detection scheme for sequencing biomolecules. DNA has been elongated by electrophoresis both in a gel and in solution, using electrical forces to move the DNA in position for reading (U.S. Pat. No. 5,538,898). However, no data were given to determine the quality of the stretching of large polymers, and the technique is limited to analyzing approximately 3 megabases at a time.
Gravitational forces have also been used to stretch DNA (U.S. Pat. No. 5,707,797; Windle (1993) Nature Genetics 5:17-21). In this technique, drops of DNA from the sodium dodecyl sulfate lysing of cells were allowed to run down a slide held at an angle. The effect of gravity was enough to stretch out the DNA, even to its over-stretched S-DNA form. The DNA was then immobilized on the slide, making processing, e.g., fluorescent labeling, prior to stretching relatively difficult.
Single-molecule DNA analytical methods which involve elongation of DNA molecule include optical mapping (Schwartz et al., 1993, Science 262:110-113; Meng et al., 1995, Nature Genet. 9:432; Jing et al., Proc. Natl. Acad. Sci. USA 95:8046-8051) and fiber-fluorescence in situ hybridization (fiber-FISH) (Bensimon et al., Science 265:2096; Michalet et al., 1997, Science 277:1518). In optical mapping, DNA molecules are elongated in a fluid sample and fixed in the elongated conformation in a gel or on a surface. Restriction digestions are then performed on the elongated and fixed DNA molecules. Ordered restriction maps are then generated by determining the size of the restriction fragments. In fiber-FISH, DNA molecules are elongated and fixed on a surface by molecular combing. Hybridization with fluorescently labeled probe sequences allows determination of sequence landmarks on the DNA molecules. Both methods require fixation of elongated molecules so that molecular lengths and/or distances between markers can be measured.
A method for measuring the length and distances between markers on DNA was developed by Kambara et al. (U.S. Pat. No. 5,356,776). This method involves fluorescently labeling a DNA molecule at both termini and/or internal sites, and moving the labeled molecule through a gel via electrophoresis. In so doing, the DNA molecule is forced into a straightened conformation. The straightened DNA molecule is transferred into a gel-free buffer, and the fluorescent labels are detected. The time interval between the detection of the two labels is used to determine the distance between them. If the two labels label the termini of the DNA molecule, the distance between the labels measures the length of the molecule. The method, which does not provide means for determining the velocity of the DNA molecule, relies on estimating the velocity of DNA from the migration rate of the DNA molecule.
Flow based single-molecule analytical methods for elongation and characterization of single macromolecules have not been widely adopted due in part to the difficulty in precise measurement of molecular characteristics, e.g., the length of the macromolecule, the distance between two landmarks on a macromolecule, etc. For example, to determine the length of an elongated macromolecule as it travels through a detection zone, e.g., a laser excitation zone, it is necessary to know the velocity of the macromolecule. The flow velocity field can be measured by various known methods, e.g., particle image velocimetry (PIV) (see, e.g., Meinhart et al., 1999, Experiments in Fluids 27: 414-419; Meinhart et al., 2000, Meas. Sci. Technol. 11:809-814). The velocities of flexible objects, such as elongated polymers, may not be the same as the flow velocities. For example, in most flows the length of a polymer may be changing as it travels along with flow. In particular, the length of a polymer may be changing as a consequence of changing flow velocity. There is therefore a need for faster, simpler, more reliable and more universally applicable methods for measuring the velocities of single elongated polymers traveling in a flow. There is also a need for more accurate methods for determining the length of single elongated polymers and/or distances between landmarks on single elongated polymers.
Citation of a reference herein shall not be construed as indicating that such reference is prior art to the present invention.